Histone mRNA concentrations are regulated in parallel with DNA synthesis. There are two classes of histone mRNA; the major class which is regulated and codes for the replication histone variants, and a minor class, which is constitutively expressed by codes for replacement histone variants. The histone mRNA levels are regulated during the cell cycle and the signals which regulate DNA and histone synthesis may be related. Three major questions will be addressed: 1. What are the nucleic acid sequences involved in regulation of histone mRNA levels. These include DNA sequences involved in regulation of transcription and mRNA sequences involved in regulation of mRNA half-life. In particular, what are the differences in gene structure between the replication and replacement histone variants which differ in their regulation. We will answer this question by constructing variant genes, introducing them into cells and monitoring their regulatory behavior. In particular we will construct hybrid genes containing different portions of replication and replacement variant histones to see which portions are important in regulation. 2. What is the signal which regulates histone mRNA levels? The levels of histone mRNA are regulated by agents which alter deoxynucleotide metabolism. The hypothesis that an unusual nucleotide regulates histone mRNA levels will be tested. Changes in intracellular nucleotides which occur concominantly with alterations in histone mRNA levels will be analyzed. A cell-free system which may be used to assay for this signal will be developed. 3. Why is one cluster of histone genes on chromosome 3 expressed in 5 to 10 times the amount of the histone genes on chromosome 13? Genes will be constructed to test for the possibility of enhancer sequences on the histone genes on chromosome 3.